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ADM3311EARSZ Datasheet(PDF) 14 Page - Analog Devices |
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ADM3311EARSZ Datasheet(HTML) 14 Page - Analog Devices |
14 / 24 page ADM3307E/ADM3310E/ADM3311E/ADM3312E/ADM3315E Data Sheet Rev. I | Page 14 of 24 A block diagram of the Green Idle circuit is shown in Figure 26. Both V+ and V− are monitored and compared to a reference voltage derived from an on-chip band gap device. If either V+ or V− fall below 7 V1, the oscillator starts up until the voltage rises above 7.25 V2. TRANSCEIVERS START/STOP START/STOP V+ V– SHUTDOWN CHARGE PUMP V– VOLTAGE COMPARATOR WITH 250mV HYSTERESIS BAND GAP VOLTAGE REFERENCE V+ VOLTAGE COMPARATOR WITH 250mV HYSTERESIS Figure 26. Block Diagram of Green Idle Circuit The operation of Green Idle for V+ under various load conditions is illustrated in Figure 27. Under light load conditions, C1 is maintained in a charged condition, and only a single oscillator pulse is required to charge up C2. Under these conditions, V+ may actually overshoot 7.25 V2 slightly. OSC V+ OVERSHOOT LIGHT LOAD 7.25V1 7V2 OSC V+ 7.25V1 7V2 OSC V+ 7.25V1 7V2 MEDIUM LOAD HEAVY LOAD 1FOR ADM3310E AND ADM3315E REPLACE WITH 6.5V. 2FOR ADM3310E AND ADM3315E REPLACE WITH 6.25V. Figure 27. Operation of Green Idle under Various Load Conditions Under medium load conditions, it may take several cycles for C2 to charge up to 7.25 V2. The average frequency of the oscillator is higher because there are more pulses in each burst and the bursts of pulses are closer together and more frequent. Under high load conditions, the oscillator is on continuously if the charge pump output cannot reach 7.25 V2. Green Idle Vs. Shutdown Shutdown mode minimizes power consumption by shutting down the charge pump altogether. In this mode, the switches in the voltage tripler are configured so V+ is connected directly to VCC. V− is zero because there is no charge pump operation to charge C5. This means there is a delay when coming out of shutdown mode before V+ and V− achieve their normal operating voltages. Green Idle maintains the transmitter supply voltages under transmitter idle conditions so this delay does not occur. Doesn’t Green Idle Increase Supply Voltage Ripple? The ripple on the output voltage of a charge pump operating in open-loop depends on three factors: the oscillator frequency, the value of the reservoir capacitor, and the load current. The value of the reservoir capacitor is fixed. Increasing the oscillator frequency decreases the ripple voltage; decreasing the oscillator frequency increases it. Increasing the load current increases the ripple voltage; decreasing the load current decreases it. The ripple voltage at light loads is naturally lower than that for high load currents. Using Green Idle, the ripple voltage is determined by the high and low thresholds of the Green Idle circuit. These are nominally 7 V1 and 7.25 V2, so the ripple is 250 mV under most load conditions. With very light load conditions, there may be some overshoot above 7.25 V2, so the ripple is slightly greater. Under heavy load conditions where the output never reaches 7.25 V2, the Green Idle circuit is inoperative and the ripple voltage is determined by the load current, the same as in a normal charge pump. What about Electromagnetic Compatibility? Green Idle does not operate with a constant oscillator frequency. As a result, the frequency and spectrum of the oscillator signal vary with load. Any radiated and conducted emissions also vary accordingly. Like other Analog Devices RS-232 transceiver products, the ADM33xxE devices feature slew rate limiting and other techniques to minimize radiated and conducted emissions. 1 For ADM3310E and ADM3315E, replace with 6.5 V. 2 For ADM3310E and ADM3315E, replace with 6.25 V. |
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